Interestingly enough, when this is broken down by time-of-arrival cohorts, the proportion coming from rural areas does not decline as could be expected because of the steady increase in urbanization in both northeastern Mexico and Mexico as a whole. It is 54 percent rural for those arriving before 1941; 51 percent in the 1941-1950 period; 59 percent in the 1951-1960 period; and 60 percent in the 1961-65 period. As we shall see later on, there are reasons for believing that the earlier migrants to Monterrey were more selective than those arriving later, and this selectivity was manifested in a greater propensity to come from urban areas, even though the country was more rural at the earlier time.
About all that can be safely generalized about the rural-urban balance of migration is: Large cities in developing areas draw migrants from urban and rural areas, with both being well represented, though not necessarily in close correspondence with national or regional rural-urban ratios. Weak as this proposition may be, it still guards us against easy assumptions that migrants are nearly all from rural areas or that there is a mechanical relationship between the national or regional rural-urban distribution and the origins of migrants.
Little is known about the factors that affect out-migration from small and medium-size cities because, as already noted, these places have been the object of less investigation than either rural areas or large cities. However, it may be presumed that large cities exercise a strong attraction for many of the inhabitants of small and medium-size cities. In most developing countries it is the large metropolitan centers that benefited most from industrialization, whereas many smaller places have remained economically stagnant. Since the inhabitants of the medium and smaller urban places are generally literate and often, via the mass media, well aware of the superior economic prospects in the large cities, they migrate in the expectation of bettering their socioeco-nomic situation. Therefore: Migrants to large cities in developing countries coming from medium or small urban places are positively selective of the populations from which they originate.
Out-Migration from Rural Areas
Turning to the subject of out-migration from rural areas, we are confronted with one of the most distinctive as well as most important features of developing societies. What makes the situation of these rural areas especially critical at this time is their increasing rate of population growth. While countries, and regions within countries, vary somewhat in this respect, virtually all of them have experienced significant reductions in mortality levels within the last generation. Because fertility has remained high, a pronounced rise in the rate of natural increase has resulted. This in turn has meant an increase, in various degrees and various ways, of population pressure on resources (24).
Not all of the people in rural areas of developing countries are peasant inabsent from the village. For further methodological details of the two studies see Caldwell (12, Ch. 1) and Balan, et al. (1 3, Ch. 1). Substantively, Ghana differs from Mexico in the predominantly male rural-urban migration, in the multilingual character of the society, in the existence of polygamy, and